This study is designed to investigate in depth the structure, function, and pathogenic significance of newly described human serum low density lipoprotein inhibitor (LDL-In) which may represent a normal physiological repressor of lymphocyte function. LDL-In will be purified, biochemically characterized, and the biologically active moiety identified. The cellular sites of action and the pathway by which LDL-In regulates immune function will be characterized. Finally, the association between immunological competence, serum levels of LDL-In, and the presence and function of cell surface receptors for LDL-In will be analyzed in man and experimental animal models, in an attempt to validate the physiological significance of LDL-In-mediated immunosuppression.